Diffusion of CH4, CO2, and Their Mixtures in AlPO4-5 Investigated by QENS Experiments and MD Simulations

Abstract

Quasi-elastic neutron scattering (QENS) measurements in combination with molecular dynamics (MD) simulations have been performed to characterize the dynamics of CH4, CO2, and binary mixtures of different compositions in the zeolite-type AlPO4-5 material. The experimental and simulated self-diffusion coefficients (Ds) for CH4 in the presence of CO2 are in very good agreement in a whole range of CO2 concentrations, showing a decreasing profile when the CO2 loading increases. Similar to the diffusion of light gases in other nanoporous materials, the experimental and simulation approaches both evidence a fast mobility for CH4 at low loading in this zeolite. Complementary to this, the MD simulations predict a slightly faster diffusivity for CH4 in binary mixtures with CO2 when compared to its behavior as a single component, which is concomitant with a speeding up of the CO2 molecules. QENS further reveals a nonmonotonous evolution of the transport diffusivity for CO2 as a function of the loading. This peculiar behavior is reproduced by MD simulations, with the minimum being shifted to a higher concentration. A deep analysis of the MD spatial densities indicates that both CO2 and CH4 experience a 1D-type normal diffusion along the AlPO4-5 channels in a hollow cylinder with a hexagonal base. Finally, QENS and MD allow the exploration of the rotational dynamics of CH4 as a pure component and in a binary mixture.